1. Field of the Invention
The present disclosure relates to a switching gate driver which is included to drive an inverter, and more particularly, to a switching gate driver to reduce an On/Off switching stress.
2. Description of the Related Art
An inverter is a device that converts an AC voltage to a DC voltage, generates an AC voltage when a switching device switches the converted DC voltage according to a PWM (Pulse Width Modulation) signal, and outputs the generated AC voltage to a load to be driven. It provides the load with an AC voltage having voltage and frequency required by a user so that the driving of the load can be precisely controlled.
An IGBT (Insulated Gate Bipolar Transistor) is normally used as a switching device included in the inverter. A switching gate driver is a circuit that controls the IGBT or MOS transistor.
FIG. 1 is a circuit diagram illustrating a gate driver. A gate driver to control most of IGBT devices used in the industry has the same construction as that illustrated in FIG. 1.
Referring to FIG. 1, the gate deriver includes gate resistors RG(on), RG(off), Rin and RGE, a capacitor C, and 2 switching devices, in order to switch an IGBT device. The gate driver receives an IGBT control signal, converts a signal into voltage levels VG+ and VG− appropriate to drive the IGBT and charges/discharges the gate of the IGBT device through the gate resistors RG(on) and RG(off). The gate voltage Vge of the IGBT device is applied according to the amount of charge accumulated at the IGBT gate, and the IGBT is turned on when the gate voltage Vge becomes higher than the driving voltage of the IGBT device.
When the gate voltage Vge of the IGBT device becomes equal to or less than the driving voltage, the IGBT device is turned off. At this time, the turn-on and turn-off time of the IGBT device is determined by the magnitudes of the resistors RG(on) and RG(off). When designing the gate resistors RG(on) and RG(off) in a small magnitude, a large spike voltage occurs between collector and emitter terminals of the IGBT device in a turn-off operation of the IGBT device due to a sudden current change occurring when the IGBT device is switched, and a large reverse recovery current of a freewheeling diode occurs in the turn-on operation of the IGBT device. On the other hand, when designing the gate resistors RG(on) and RG(off) in a large magnitude, current change time becomes long so that a switching loss is increased.
FIGS. 2 to 5 are circuit diagrams illustrating a variety of embodiments of the gate driver illustrated in FIG. 1.
FIG. 2 illustrates a case that includes resistors RG(on) and RG(off), and FIG. 3 illustrates a case that embodies resistors RG(on) and RG(off) as one resistor. FIG. 3 illustrates a charge current path when the IGBT device is turned on, and FIG. 4 illustrates a discharge current path when the IGBT device is turned off.
As described above, the gate driver in the art is constructed of a MOS transistor providing the IGBT device with a driving power or a totem-pole circuit, and a gate resistor RG that controls an IGBT gate charge/discharge current. The gate resistor RG may be separately constructed of a gate resistor RG(on) that controls the gate charge current of the IGBT device when the IGBT device is turned on and a gate resistor RG(off) that controls the gate discharge current of the IGBT device when the IGBT device is turned off, as illustrated in FIG. 2, or may be constructed of one gate resistor without dividing turn on/turn off operations of the IGBT device, as illustrated in FIG. 3.
The gate charge/discharge current of the IGBT device is charged through the gate resistor RG(on) when the IGBT device is turned on, as illustrated in FIG. 4 and discharged through the gate resistor RG(off) when the IGBT device is turned off, as illustrated in FIG. 5. The gate resistor RG is predetermined as an appropriate value in consideration of an IGBT spike voltage occurring when the IGBT device is turned off, a reverse recovery current of a freewheeling diode occurring when the IGBT device is turned on, and a switching loss.
The spike voltage of the IGBT device determined by the gate resistor RG and the reverse recovery current of a freewheeling diode have a complementary relationship with respect to a switching loss. At this time, when the gate resistor RG is designed too large, gate charge/discharge time for the IGBT becomes long, and an IGBT spike voltage of a collector-emitter voltage Vce of the IGBT device occurring when the IGBT device is turned off or turned on and a reverse recovery current of a freewheeling diode are decreased. However, a switching loss is increased.
When designing the gate resistor RG to be small, gate charge/discharge time for the IGBT device becomes short so that a switching loss is decreased. However, an IGBT spike voltage of a collector-emitter voltage Vce of the IGBT device and a reverse recovery current of a freewheeling diode are increased. Since the gate resistor RG uses a fixed value designed, gate charge/discharge time of the IGBT device is constant.